1. Field of the Invention
Method for optimizing a half-tone representation on a photoconductor of electrophotographic printer and copier apparatuses
The present invention relates to a method for optimizing a half-tone representation by optimization of the toner application intensity on a photoconductor of electrophotographic printer and copier apparatuses.
2. Description of the Related Art
Users make extremely high quality demands of printouts and copies produced by means of electrophotographic printer or, respectively, copier apparatuses. In order to meet these high demands, it is necessary to reduce the ranges of tolerance permissible in electrophotographic processes.
For example, in electrophotographic printer and copier apparatuses individual pages or endless strips of paper are printed by producing a latent image on a photoconductor, preferably fashioned as a drum. For this purpose, a photoconductor is charged to a defined charge potential, and subsequently, dependent on the method used, the regions that appear white or black in the printout are exposed. The exposed regions then comprise a discharge potential that is lower in relation to the charge potential. The produced latent image is then developed by applying toner to the exposed or unexposed regions, dependent on the method used, so that these regions appear black in the printout.
The toner standardly used is preferably a two-component toner consisting of a carrier component and microtoner. In turn, the two-component toner is charged positive or negative dependent on the method used.
The image developed on the photoconductor is subsequently transferred onto paper or onto another recording medium, and, in a fixing station, is subsequently melted into the recording medium by heating, or is bonded therewith by means of adhesive forces that arise during the melting of the toner image.
After the transfer of the image from the photoconductor onto the recording medium, the photoconductor is cleaned and fully discharged, in preparation for the production of the next image.
The differences in density of different printouts, caused by fluctuations of the process parameters of printer and copier apparatuses, become particularly clear when a large number of identical images are produced in succession. If these images are for example images with gray surfaces, comprising a fine gray value gradation, even small fluctuations in the gray value are perceived by the human eye.
Such fluctuations can for example be caused by process parameters such as the charge potential to which the photoconductor is charged at the beginning of each print process, by the discharge potential attained by certain regions of the photoconductor after the exposure, and by fluctuations in the intensity of exposure. The charge and/or discharge potential of the photoconductor can be dependent in particular on the producing charge, the duration of use, the temperature and the cyclical loading of the photoconductor.
A toner deposit intensity, i.e. the quantity of toner deposited in a region to be inked on the photoconductor, is essentially dependent on the air humidity and the toner concentration in the two-component toner, i.e. the mixing ratio between the microtoner and the carrier components. In addition, the toner deposit intensity is dependent on the triboelectric state of excitation of the two-component toner, which is for example in turn dependent on the temperature, the air humidity, the duration of use, the intensity and the duration of the thorough mixing of the two-component toner, as well as on the quantity of fresh toner supplied to the mixer.
For limiting the above-described parameter fluctuations in electrophotographic processes, it is known to keep the photoconductor temperature constant, to regulate the charge potential of the photoconductor, to regulate the discharge depth, i.e. the difference between the charge and discharge potential of the photoconductor, to keep constant the exposure energy for the production of the discharge potential, to regulate the toner concentration in the two-component developer or for example to carry out a toner mark regulation. Toner marks are here understood as regions that are arranged outside the print image on the photoconductor and are exposed and developed for process monitoring and control.
The above-described regulations for the reduction of the parameter fluctuations are carried out either individually or in combinations. Individually, the above regulation methods effect only a partial stabilization, which is however not sufficient for the high demands made with respect to the required print quality, in particular given a half-tone representation.
From the German Published Patent application DE-A1-38 43 672, a method is known for optimizing a toner deposition intensity in copier apparatuses that operate in analog fashion, in which method, dependent on an optical density of a toner mark, a bias potential and/or a toner concentration is modified. As a toner mark, an unrastered "image pattern" with full-surface inking is used.
Due to the use of a toner mark with full-surface inking, in connection with the analog functioning of the copier apparatus, this regulation is not suitable for the optimization of a half-tone image.
If, in contrast, a coarsely gridded toner mark is used that comprises, in contrast to the full-surface toner mark, both inked and non-inked regions, a stable regulation of the toner deposition intensity can be achieved only if high-ohmic two-component developers are used. Since high-ohmic two-component developers cannot be used with modern conductive developer brushes, this method cannot be used in modern printer or copier apparatuses.
In addition, for the regulation of the electrophotographic process in electrophotographic apparatuses, it is generally known to scan raster toner marks with the aid of sensors, and to control process parameters dependent on the optical density thereof. Thus, in U.S. Pat. No. 4,999,673 the integral density of a toner mark is determined via a densitometer, and the toner concentration or another process parameter is modified accordingly. In U.S. Pat. No. 4,949,105, toner marks are developed in a known manner, and the integral density thereof is used to control the process parameters. This principle is also known from U.S. Pat. No. 5,400,120, with the use of various toner marks.
The known toner marks are specifically constructed or, respectively, suited for regulating an individual process parameter. Various differently constructed toner marks are used for the regulation of the various parameters of the overall process.